Usermin webmail - cannot send, connection closed by server

I've changed default port ("listen to") from default port to a different one (4087) for security reasons
in Webmin - > Usermin Configuration
I don't know if it is the reason that I cannot send email from Usermin (when I do - after hitting "send" button it says "Connection closed by server") I have never send any mail before via webmin, so i dont know if changing the port is the reason
Anything else I can change/fix to make it work?

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Comments

What messages do you see in /var/usermin/miniserv.error whenever that occurs?

"Bad request: This web server is running in SSL mode. Try the URL https://... instead" Hmm, maybe I wasn't using Usermin in https:// mode?

Yes, you definately need to use an https:// URL. Also, make sure the new port isn't blocked on your system's firewall.